Test of Variational Transition State Theory and Multidimensional Semiclassical Transmission Coefficient Methods against Accurate Quantal Rate Constants for H + H₂/HD, D + H₂, and O + H₂/D₂/HD, Including Intra- and Intermolecular Kinetic Isotope Effects

Rate constants and kinetic isotope effects for the title reactions have been calculated by using accurate quantum dynamical methods and used to test the accuracy of corresponding rate constants from conventional and variational transition state theory. The quantum dynamical rate constants are estimated to be within 35% of the exact rate constants for the potential surfaces chosen for this comparison. For all the reactions considered, the conventional and variational transition state theory rate constants with unit transmission coefficient are found to be very close to each other (better than 7%) but in poor agreement with the accurate quantum results (off by factors of 6–22 at 300 K). This indicates that although variational effects are small, tunnelling makes a very important contribution to the rate constants, and it is found that this tunnelling contribution is described quantitatively for all the reactions considered with use of the least action ground state (LAG) transmission coefficient. The combination of improved canonical variational theory (ICVT) and LAG yields rate constants which have an average error (considering all the reactions and temperatures studied) of 15% compared to the accurate quantum rate constants, and in only one case (D + H₂ at 200 K) does the ICVT/LAG rate constant differ by more than 35% from the accurate value. The comparison of ICVT/LAG kinetic isotope effects is found to be similarly good, with the worst comparisons occurring for intramolecular (X + HD) isotope ratios.